1. Field of the Invention
The present invention generally relates to a liquid crystal display (LCD), and especially relates to a liquid crystal display panel capable of displaying color images.
2. Description of the Related Art
Generally, a LCD displays a desired image on a front display area by irradiating a backlight from the rear side of the LCD through a liquid crystal layer, the orientation of which is controlled. At this instance, in order to suppress leakage of excessive light from the circumference of the display area of the LCD, it is common to provide a black frame area around the circumference of the display area.
FIG. 1 shows a typical view of the LCD panel. The LCD panel includes a display area 102 for displaying an image, and a frame area 104 that is provided around the circumference of the display area 102. The frame area 104 usually has a width (w) of about 2 mm, for example, varying by uses.
FIG. 2 shows a cross-sectional view of a conventional LCD panel, being the first example, which includes a display area 202 and a frame area 204. The cross section illustrated is near the boundary of the display area 202 and the frame area 204, and the cross section can be considered to show the portion indicated by reference number 108 of FIG. 1. The conventional LCD panel includes thin film transistors (TFTs) 208 that are formed on a glass substrate 206. A pixel electrode 210 is connected to each of the TFTs 208 that control charging and discharging of pixels of the display area 202. Corresponding to each of the pixel electrodes 210, color filters 214 are formed on another glass substrate 212, where the color filters consist of three parts, each being differently colored, namely, red (R), green (G), and blue (B). The width of each of the color filters 214 varies with uses; however, the width is about 100 micrometers per color, for example. The color filters 214 are connected to a common electrode 216 (sometimes called a counter-electrode) that is connected to a reference voltage such as ground potential. The pixel electrode 210 and the common electrode 216 are made from, e.g., ITO (indium tin oxide). Further, a shading film 218 called a black matrix layer (BM layer) is formed on the frame area 204 side of the glass substrate 212. The shading film 218 is made from, e.g., chromium (Cr), the thickness of which ranges between 0.1 and 0.2 micrometers. The glass substrates 206 and 212 are supported by a sealing material 220 and bead spacers 222 that are provided between the glass substrates, and liquid crystal is enclosed between the glass substrates.
In the first example of the conventional LCD panel, the light for display (backlight) is irradiated from the side of the glass substrate 206. In the display area 202, an image is displayed on the side of the glass substrate 212 with the TFTs 208 controlling the voltage between the electrodes 210 and 216 such that the orientation of liquid crystal molecules is controlled. At this instance, although light is also provided to the frame area 204, the light is reflected or shaded by the shading film 218, and the light does not reach the glass substrate 212. In this manner, unnecessary light, which does not contribute to an image, is prevented from leaking to the front, i.e., the glass substrate 212 side.
FIG. 3 shows a partial cross-sectional view of the second example of the conventional LCD panel. The same reference number is given to each element that is the same as explained in FIG. 2. The second example differs from the first example in that a resin black layer having a thickness of between 1 and 1.5 micrometer forms a shading film 219. Further, a protection layer (overcoat layer) 217 made of acrylic resin is formed between the counter-electrode 216 and the color filters 214, and on the shading film 219.
FIG. 4 shows a partial cross-sectional view of the third example of the conventional LCD panel. The same reference number is given to each element that is the same as explained in FIG. 2. The third example differs from the first and second examples in that the shading film 219 made of resin and the color filters 214 are prepared on the side of the TFTs 208. This structure is often called CFonTFT (Color Filter on Thin Film Transistor).
According to the LCD panels as shown in
FIG. 2 through FIG. 4, the shading film (BM layer) 218 or 219, as applicable, made of metal or resin, respectively, is provided in the frame area 204 that surrounds the display area 202. In this manner, excessive light is prevented from leaking. However, these LCD panels need to form the shading film in the frame area, which requires a manufacturing process in addition to making the display area. The additional manufacturing process, raising the manufacturing cost, is the problem with the LCD panels shown in FIG. 2 through FIG. 4.
FIG. 5 shows a partial cross-sectional view of the fourth example of the conventional LCD panels. The same reference number is given to each element that is the same as explained in FIG. 2. In the fourth example, three color filters 215 that are layered are provided to the frame area 204. The backlight that enters the frame area is sharply attenuated by passing through three color filters 215, such that unnecessary light is prevented from penetrating toward the substrate 212. The material and the thickness of each of the three color filters 215 are the same as the color filters 214 of the display area 202. Therefore, the color filters 215 of the frame area 204 and the color filters 214 of the display area 202 can be simultaneously manufactured. Accordingly, the number of manufacturing processes is reduced as compared with the examples shown in FIG. 2 through FIG. 4. The LCD panel having the structure of the fourth example is indicated by patent reference 1.
[Patent reference 1]
JP,2000-29014, A
[Problem(s) to be Solved by the Invention]
Nevertheless, the structure as shown in FIG. 5 has another problem in that a level difference d is generated between the display area 202 and the frame area 204. Although the level difference varies with implementation, when the thickness of the color filters is set at 1.8 micrometers, for example, the level difference d can become as great as 2 to 3 micrometers. Due to the level difference d, the interval (cell gap) of the LCD panel becomes greater in the circumference section (frame area) than the central part (display area). For this reason, brightness in the circumference section becomes higher than the central part, causing display spots, and degrading image quality of the display area 202 near the frame area 204.
Further, since the level difference d makes the flow passage for the liquid crystal narrow in the frame area 204, as compared with the display area 202, when the liquid crystal is to fill the space between the two glass substrates 206 and 212, the flowing speed of the liquid crystal is decreased. This decreases production throughput in the manufacturing process of the LCD panel. Furthermore, the level difference may make it difficult for the liquid crystal to fill all the corners.
FIG. 6 shows a partial cross-sectional view of the fifth example of the conventional LCD panels. The same reference number is given to each element that is the same as explained in FIG. 2. In the fifth example, only one color filter 215 (B) is formed in the frame area, the color filter (B) being blue-colored with low permeability. According to this example, while the problem about the level difference as described in connection with the example of FIG. 5 is solved, the shading effect is insufficient because only one color filter 215 is used, and the ability to show the frame in black, i.e., the frame performance, is degraded. In the fifth example, the frame area 202 appears bluish rather than pure black. Specifically, the optical-density (OD) value (the higher the OD value, the higher the shading ability) of the shading film 218 that consists of metal or resin in the cases of FIG. 2 through FIG. 4 ranges between 3 and 4. On the other hand, the OD value in the case of FIG. 6 is 2.0 or less. That is, in the fifth example shown in FIG. 6, the frame performance is sacrificed for solving the problem of the level difference.